Loads and Loads of Characters: The series started off with thirty-one students, one teacher, one principal, and two advisors. It has since expanded with new characters coming in from the magical community.
The character page had to be split in seven because it broke the folder system. Twice.
Lock Down: Why are there blast doors in Library Island?
Look What I Can Do Now: Negi pulls out no less than five new techniques as trump cards during his fight with Rakan. Trump card number 3 was basically an upgrade to trump card number 2, and trump card number 4 wasn't even used except to trick Rakan into helping Negi set up for trump card number 5. Kotaro also demonstrates a brand new form and a few new techniques during that fight.
Lost in Translation: The official Del Rey English release removed foreshadowing for jokes and innuendo in several places, and made it so Negi was at the bottom of his class instead of the top, among other things. The Omnibus re-releases fixed this.
Love Is in the Air: When Asuna force-feeds Negi his own love potion; particularly notable in Negima!
Love Potion: See above. Also, the love chocolates which Asuna mistook for just plain chocolate that caused her to become infatuated with Negi for a whole day, though it's specifically pointed out that magic isn't supposed to work on Asuna, making it questionable how much effect they had.